This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-157030, filed May 25, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an apparatus and method for heating a substrate.
2. Description of the Related Art
In a photolithographic step for a semiconductor device, various types of heating processes are performed including prebaking for vaporizing a solvent from a resist solution applied to a semiconductor wafer and post-exposure baking (PEB) for improving the sensitivity of a chemically amplified resist film after light exposure.
Such a heating process is performed by mounting a wafer on a hot plate of a heating apparatus, so that thermal energy is directly transferred from the hot plate to the wafer while the hot air in the process chamber is removed by an exhaust cover. As a conventional hot plate, a thick ceramic plate of disk-like form is used (which is manufactured by sintering powders such as silicon carbide or aluminum nitride to form a single piece). A resistance heater is embedded in the hot plate for heating the wafer to a predetermined target temperature. In the baking process of general photolithography, a wafer is heated to within the range of about 100 to 200xc2x0 C.
However, recently, a baking process for heating a wafer to a further higher temperature range has come into use with diversification of manufacturing processes for semiconductor devices. In such a baking process, a wafer is heated to, for example, about 700xc2x0 C. However, if a conventional hot plate is baked in such a high-temperature baking process, the hot plate becomes warped. More specifically, both ends of the plate become warped as shown in FIG. 15. When a conventional hot plate was experimentally heated to 700xc2x0 C., it was confirmed that the hot plate cannot withstand the warping and sometimes breaks. In addition, when a wafer is loaded into or unloaded from a heating process chamber, ambient air flows into the heating process chamber to change the inner temperature thereof by about 100xc2x0 C. Since heating and cooling are repeated, the hot plate repeatedly expands and contracts, so that the quality of the hot plate deteriorates in a short time.
When the hot plate distorts, the heat is transferred non-uniformly from the hot plate to the wafer, with the result that the wafer is non-uniformly heated. The wafer becomes wavy and distorted like a saddle-back, as shown in FIG. 16. Such a distortion is undesirable since it decreases dimensional accuracy and the yield of a semiconductor device. In particular, these days, the size of wafers has been increased in order to improve productivity, etc. In view of this tendency, the distortion of wafers becomes a serious problem. It is therefore desired to decrease the distortion of wafers.
The hot plate to be used in the heating process is desirably thin in view of heat response (quickly heating and cooling). Nevertheless, the thickness of the hot plate has not actually been reduced because a minimum strength is required to prevent breakage of the hot plate as mentioned above. Particularly, in the case where the heating process is performed at a higher temperature, it is difficult to reduce the thickness of the hot plate.
An object of the present invention is to provide a heating apparatus and method using a thinner hot plate for heating a substrate while preventing a crack of the hot plate by suppressing the distortion of the hot plate.
According to the present invention, there is provided a heating apparatus for heating a substrate to be processed by photolithography, comprising
a central hot plate having a heat-generating surface which faces a center portion of a lower surface of the substrate and heating the center portion of the substrate;
a plurality of segment hot plates provided so as to surround a periphery of the central hot plate in a plan view, having a heat generating surface facing a peripheral portion of the lower surface of the substrate and heating a periphery of the substrate;
a hot plate support member supporting the central hot plate and the segment hot plates;
a substrate support member supporting the substrate so as to face the central hot plate and the segment hot plates in a close proximity without being in contact with the central hot plate and the segment hot plates; and
a power supply for supplying electricity to the central hot plate and each of the segment hot plates.
According to the present invention, there is provided a method of heating a substrate to be processed by photolithography, comprising:
(a) preparing a central hot plate having a heat-generating surface which faces a center portion of a lower surface of the substrate and a plurality of segment hot plates each having a heat-generating surface which faces a peripheral portion of the lower surface of the substrate;
(b) arranging the plurality of segment hot plates around the central hot plate; forming an annular clearance between the segment hot plates and the central hot plate, inserting a plurality of support pins into the annular clearance from the bottom and allowing the support pins to protrude upward, supporting the substrate by the support pins without being in contact with the segment hot plates and the central hot plate; and
(c) heating the center portion of the lower surface by the central hot plate, heating the peripheral portion of the substrate by the plurality of segment hot plates, thereby heating the entire substrate to a predetermined target temperature.
By combining a plurality of segment hot plates with the central hot plate as mentioned above, each of the hot plates is reduced in size. As a result, the distortion of the entire hot plate assembly is greatly reduced to maintain it flat. Since the distortion of the hot plate itself is suppressed, the thickness of the hot plate can be reduced. When the hot plate thus reduced in thickness is used, high heat response (quickly heating and cooling) of the hot plate is improved. It is therefore possible to quickly increase or decrease the temperature of the hot plate.
Furthermore, since the structures of the segment hot plates and the central hot plate are simplified, the manufacturing process of the hot plate becomes simple, reducing the manufacturing cost. Furthermore, the performances of the segment hot plates and the central hot plate can be independently evaluated. In the case, if a defective plate is included in the hot plate assembly, it is sufficient to discard only the defective plate. The yield is therefore improved. In the event where the hot plate assembly becomes out of order during use, it is not necessary to discard the entire hot plate assembly but necessary to replace only a broken part with a new one. For this reason, the maintenance cost is greatly reduced.
The central hot plate preferably has a circular from in a plan view. The segment hot plates are formed by dividing a ring-form peripheral portion surrounding the central hot plate into four regions and preferably has a fan-shape in a plan view. Note that it is most preferable that the form of the central hot plate should be a perfect circle in a plan view. However, the central hot plate may be a polygon such as a right hexagon, right octagon, right decagon, and right dodecagon. The shape of the segment hot plate in a plan view is most preferably a fan-shape, however, a trapezoid may be acceptable.
Furthermore, it is preferable to have a temperature control unit for controlling the amount of heat generated from each of the segment hot plates by controlling the amount of electricity to be supplied from a power supply to each of the segment hot plates. By controlling the heating operation of each segment hot plate in this manner, it is possible to minutely control the temperature of the substrate peripheral portion to significantly reduce the heat distortion amount of the substrate peripheral portion.
It is further preferable to have a thermo sensor provided to each of the segment hot plate and the substrate in noncontact therewith, for detecting at least one of temperatures of the lower and upper surfaces with respect to (the hot plate and) the substrate. By using the non-contact thermo sensor, the thickness of the hot plate can be reduced. In the event where a part of the hot plate becomes out of order, the same thermo sensor may be used continuously without being replaced.
The thermo sensor is preferably arranged immediately below the radial clearance formed between adjacent segment hot plates. This is because the temperature of the lower surface of the substrate, which receives radiant heat energy from the heat-generating surface of the hot plate can be directly detected by the thermo sensor. Note that the thermo sensor may be arranged above the hot plate assembly. This is because the upper surface temperature of the substrate can be detected by the thermo sensor thus arranged. Alternatively, by using the lower temperature sensor and the upper temperature sensor in combination, both the temperature of the upper and lower surfaces of the substrate may be detected.
Furthermore, it is preferable to have an up-and-down movement mechanism for moving up and down the segment hot plates together with the hot plate support member. By controlling the distance between the substrate and the segment hot plates by the up-and-down moving mechanism, it is possible to minutely control the heat amount given to the substrate from the segment hot plates. As a result, the substrate is more uniformly heated and the heat distortion of the substrate is further efficiently suppressed.
It is desirable that the substrate support member be independently and discretely formed from the hot plate support member such that the load of the substrate should not be applied to the hot plate assembly. By virtue of this structure, the distortion of the hot plate due to the load of the substrate can be suppressed. In addition, since the load to be applied to the hot plate is reduced, the thickness of the hot plate can be reduced.
An annular clearance is formed between the central hot plate and the segment hot plates. The substrate support member is formed of a plurality of support pins arranged within the annular clearance. It is further preferable to have a rotation-driving mechanism for rotating a plurality of support pins. Since the substrate can be rotated by this construct during the baking process, even if temperatures slightly differ between the segment hot plates, the difference of heat amounts received by the substrate can be cancelled out. As a result, the substrate can be heated uniformly. Furthermore, the alignment operation of the substrate usually performed by a conventional method can be omitted.
It is also preferable that a shielding member should be provided below the hot plate assembly for inhibiting cool air from passing through the annular clearance. Such a shielding member is desirably attached to the support pins and a ring-form seal film made of a polyimide resin. It is also desirable that numerous small holes are formed in the ring-form seal film to prevent a local temperature change of the substrate above the hot plate by air flowing through the annular clearance.
The central hot plate has an outer peripheral edge surface formed obliquely at a predetermined angle with respect to the heat-generating surface. The annular clearance obliquely formed between the outer peripheral edge surfaces of the central hot plate and the segment hot plates, that is, the inner peripheral edge surface of each of the segment hot plates, which is obliquely formed at a predetermined angle with respect to the heat-generating surface. It is preferable that the outer peripheral edge surface of the central hot plate should be overlapped with the inner peripheral edge surfaces of the segment hot plates in a plan view. Heat energy is emitted from both the outer peripheral edge surface and the inner peripheral edge surfaces in the same manner as from the heat-generating surface. However, the lower surface of the substrate cannot be seen through the annular clearance obliquely formed, so that the substrate can be efficiently prevented from being heated nonuniformly.
Furthermore, the segment hot plates may preferably comprise a plurality of inner segment hot plates concentrically arranged around the central hot plate and a plurality of outer segment hot plates concentrically arranged around the inner segment hot plates. By increasing the number of the segment hot plate in this manner, it is possible to minutely control the temperature of the substrate peripheral portion.
Furthermore, a first radial clearance is formed between adjacent inner peripheral segment hot plates and a second radial clearance is formed between adjacent outer peripheral segment hot plates. The second radial clearance is preferably formed away from the first clearance so as not to position on the extension line of the first radial clearance. In this case, it is preferable that a thermo sensor should be arranged immediately below at least one of the first and second radial clearances without being in contact with the segment hot plates, for detecting the temperature of the lower surface of the substrate. It is therefore possible to directly detect the temperature of the lower surface of the substrate, which receives radiant heat energy from the heat-generating surface of the hot plate and control the heating temperature more accurately.
The diameter of the central hot plate preferably falls within the range of 20 to 60% of the diameter of the entire hot plate assembly and more preferably 40-50%.